ECT inspection is commonly used in NDT/NDI applications to detect flaws in surfaces of manufactured components fabricated out of conductive materials, such as steel bars, tubes and pipes. ECT is often used to inspect components for automotive, aeronautic and energy industries. Over the years, ECT sensors have been designed with different configurations and patterns to suit for different applications.
Various ECT systems have heretofore been provided for the detection of cracks and or other flaws in a part under test. In general, such systems include field producing means such as a coil connected to an AC source to generate Eddy currents in a part and a sensing means to sense the field produced by the Eddy currents. The sensing means may be a separate coil, a Hall probe, or any other field responsive device, or the coil of the field-producing means may also be used to sense the EC-induced field, by measuring the effective impedance thereof.
One of the challenges of performing an ECT inspection is getting sufficient eddy current field strength in the region of interest within the material. Another challenge is keeping the field away from nonrelevant features of the test component. The impedance change caused by nonrelevant features can complicate the interpretation of the signal. Probe shielding and loading are sometimes used to limit the spread and concentrate the magnetic field of the coil. Of course, if the magnetic field is concentrated near the coil, the eddy currents will also be concentrated in this area.
Eddy current probes are most often shielded using magnetic shielding or eddy current shielding. Magnetically shielded probes have their coil surrounded by a ring of ferrite or other material with high permeability and low conductivity. The ferrite creates an area of low magnetic reluctance and the probe's magnetic field is concentrated in this area rather than spreading beyond the shielding. This concentrates the magnetic field into a tighter area around the coil.
Eddy current shielding uses a ring of highly conductive but nonmagnetic material, usually copper, to surround the coil. The portion of the coil's magnetic field that cuts across the shielding will generate eddy currents in the shielding material rather than in the nonrelevant features outside of the shielded area. The higher the frequency of the current used to drive the probe, the more effective the shielding will be due to the skin effect in the shielding material.
While the shielding methods mentioned above are suitable solutions for conventional eddy current coils, they are not adapted for coils built on printed circuit boards. In this case, a magnetic shield is not compatible with the manufacturing processes. As for the conductive shield, its effectiveness would be strongly tied to test frequency because copper layers on printed circuit boards are typically very thin (in general approximately 18 micrometer thick for a flexible printed circuit board with finely etched traces). Considering the depth of penetration of the eddy currents, such thin shield would limit the use of the probe to very high frequencies such as 10 MHz and more.
Another limitation of conventional shields is the inability to overlap shielded coils over the same physical position, thus limiting the available coils configurations. Even if the conventional shields were made very thin, it would not be possible to successfully stack them because the shield of a first coil would interact with a second overlapped shielded coil in such way that none of these coils will operate properly.
Yet another limitation of conventional shield is their inability to operate properly at DC or very low frequency, such as required for remote field (RFT) or magnetic flux leakage (MFL) test methods